1. Field of the Invention
The present invention relates to an internal combustion engine provided with a direct injection-type injector for directly injecting fuel into a cylinder of the engine.
2. Related Art
A conventional internal combustion engine (which may be called merely “engine” hereinlater) of this kind is, for example, disclosed in Japanese Patent Laid-open (KOKAI) Publication No. HEI 7-269394 (reference 1) and HEI 11-132076 (reference 2).
In the engine disclosed in the above reference 1 is a direct injection system in which a fuel is directly injected into a cylinder, and a fuel pressure is changed in accordance with an operation condition of the engine. The fuel is injected at a high fuel pressure at a time of a high engine revolution speed and, on the other hand, at a low fuel pressure at a time of low engine revolution speed. According to the change in the fuel pressure, a relationship between a fuel injection period (or fuel injection time) and a fuel injection quantity varies, and in the case of the high fuel pressure, the fuel injection quantity is excessively changed even when there is a small change in the fuel injection period, thereby causing deterioration in the operational performance of the engine.
Then, in such engine, the fuel pressure is set in accordance with an operating condition, and an actual fuel injection time period is calculated by multiplying a correction amount by fuel pressure (CFP) by a basic fuel injection period determined from an intake air mass and an engine revolution speed (which may be called merely engine revolution hereinafter) to thereby set the CFP to be gradually reduced in accordance with the increasing of the fuel pressure.
On the other hand, in the engine disclosed in the above prior art reference 2, a required fuel injection quantity (Qall) is calculated from an engine revolution speed and an engine load (degree of accelerator opening), and the minimum value of this required fuel injection quantity (Qall) is limited to a minimum value (Qmin) set in response to the respective fuel pressures. That is, in a case of the required fuel injection quantity (Qall) being less than the minimum value (Qmin) set in response to the fuel pressure, the fuel injection quantity is stabilized by making coincident this required fuel injection quantity with the minimum value (Qmin), and on the other hand, in a case of the required fuel injection quantity (Qall) being more than the minimum value (Qmin), the required fuel injection quantity is applied as it is as the fuel injection quantity. At this time, an invalid injection period may be set in response to the fuel pressure.
However, in the prior art technology of the reference 1, the correction based on the fuel pressure is carried out by using one coefficient map, by which high precision control of the fuel injection quantity cannot be expected.
Furthermore, the prior art technology of the reference 2 relates to a case where the required fuel injection quantity is in the level of small quantity, and in fact, it is disclosed that “if the required fuel injection quantity (Qall) is less than the minimum value (Qmin), the fuel injection is controlled and limited to the minimum value (Qmin).” The reference 2 merely explains about control and limitation in a local narrow region, and on the other hand, it is also disclosed that the invalid injection period is set in response to the fuel pressure, but such setting method or process is not comprehended from the disclosure even by a person skilled in the art.
According to the above prior art technology, in summary, a highly accurate fuel injection quantity control in an overall operation region or area of the engine cannot be expected.